Optical element for lateral light spreading in edge-lit displays and system using same

ABSTRACT

An illumination light unit has at least one light source that generates illumination light. The unit also includes a reflecting cavity having one or more reflectors and a controlled transmission mirror disposed at an output of the reflecting cavity. The controlled transmission mirror includes an input coupling element, an output coupling element and a first multilayer reflector disposed between the input and output coupling elements. At least some of the illumination light is reflected within the reflecting cavity by the one or more reflectors and is transmitted out of the reflecting cavity through the controlled transmission mirror. The illumination light unit may be used for generating light for space lighting, or for illuminating a display. For example, the unit may be used in a backlight to illuminate a lightguide placed behind a display panel.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/167,003, filed on Jun. 24, 2005, published as U.S. 2006/0290844, nowallowed.

This application is related to the following applications, all of whichare incorporated herein by reference: U.S. Pat. No. 7,903,194; U.S. Pat.No. 7,322,731; U.S. Patent Application Publication No. 2006/0290843; andU.S. Patent Application Publication No. 2006/0290845.

FIELD OF THE INVENTION

The invention relates to optical lighting and displays, and moreparticularly to signs and display systems that are illuminated byedge-lit backlights.

BACKGROUND

Liquid crystal displays (LCDs) are optical displays used in devices suchas laptop computers, hand-held calculators, digital watches andtelevisions. Some LCDs, for example used in as laptop computers, cellphones, and some smaller computer monitor and television screens, areilluminated from behind using a backlight that has a number of lightsources positioned to the side of the display panel. The light is guidedfrom the light sources using a light guide that is positioned behind thedisplay. The light guide typically includes some arrangement forextracting the light from the light guide and directing the lighttowards the display panel. This arrangement is commonly referred to asan edge-lit display.

One important aspect of the backlight is that the light illuminating thedisplay panel should be uniformly bright. Illuminance uniformity isparticularly a problem when the light sources used at the edge of thebacklight are point sources, for example LEDs. The light guide istypically designed to spread the light within the light guide so thatthe display has no dark areas. This problem is less acute when extendedlight sources are employed, for example, fluorescent tubes, although itis still necessary to ensure that the amount of light extracted per unitarea is uniform across the display.

SUMMARY OF THE INVENTION

One embodiment of the invention is directed to an optical system thathas an image-forming panel having an illumination side and a viewingside, and a light guide disposed to the illumination side of theimage-forming panel. An illumination light unit has one or more lightsources capable of generating illumination light and a controlledtransmission mirror arranged to receive the illumination light from theone or more light sources. The controlled transmission mirror has aninput coupling element, an output coupling element and a firstmultilayer reflector disposed between the input and output couplingelements. The input coupling element redirects at least some of theillumination light incident thereon in a direction substantiallyperpendicular to the first multilayer reflector into a direction that istransmitted through the multilayer reflector to the output couplingelement. Illumination light from the output coupling element is coupledinto the light guide.

Another embodiment of the invention is directed to an illumination lightunit, that has at least a first light source capable of generatingillumination light and a reflecting cavity having one or more reflectorsand a controlled transmission mirror disposed at an output of thereflecting cavity. The controlled transmission mirror includes an inputcoupling element, an output coupling element and a first multilayerreflector disposed between the input and output coupling elements. Atleast some of the illumination light from the at least a first lightsource is reflected within the reflecting cavity by the one or morereflectors and is transmitted out of the reflecting cavity through thecontrolled transmission mirror.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The following figures and detailed description moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 schematically illustrates an edge-lit liquid crystal display(LCD) device having a backlight that uses an illumination light unit,according to principles of the present invention;

FIG. 2A schematically illustrates a cross section through an exemplaryembodiment of an illumination light unit according to principles of thepresent invention;

FIG. 2B schematically illustrates a cross section through anotherexemplary embodiment of an illumination light unit according toprinciples of the present invention;

FIGS. 3A-3D schematically illustrate cross-sectional views of differentembodiments of input coupling elements for a controlled transmissionmirror, according to principles of the present invention;

FIGS. 4A-4D schematically illustrate cross-sectional views of differentembodiments of output coupling elements for a controlled transmissionmirror, according to principles of the present invention;

FIG. 5A schematically illustrates a cross-sectional view of anembodiment of a polarization sensitive controlled transmission mirror,according to principles of the present invention;

FIGS. 5B and 5C schematically illustrate different embodiments ofpolarization-sensitive output coupling elements according to principlesof the present invention;

FIGS. 6A and 6B schematically illustrate an embodiment of anillumination light unit having a controlled transmission mirror,according to principles of the present invention;

FIG. 7A schematically illustrates another embodiment of an illuminationlight unit having a controlled transmission mirror, according toprinciples of the present invention;

FIG. 7B schematically illustrates an exemplary embodiment of a backlightthat uses an illumination light unit, according to principles of thepresent invention;

FIGS. 8A and 8B schematically illustrate other embodiments ofillumination light units having a controlled transmission mirror,according to principles of the present invention;

FIGS. 9A and 9B schematically illustrate another exemplary embodiment ofan illumination light unit having a controlled transmission mirror,according to principles of the present invention;

FIGS. 10A and 10B schematically illustrate another exemplary embodimentof an illumination light unit having a controlled transmission mirror,according to principles of the present invention; and

FIGS. 11A-11C schematically illustrate more exemplary embodiments ofillumination light units having a controlled transmission mirror,according to principles of the present invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The present invention is applicable to illuminated signs and displays,such as liquid crystal displays (LCDs, or LC displays), and isparticularly applicable to LCDs that are illuminated using light sourcesthat are placed to the side of the display panel, for example as areused in cell phones, portable DVD players, laptop computer displays andthe like.

A schematic exploded view of an exemplary embodiment of an edge-litdisplay device 100 is presented in FIG. 1. In this exemplary embodiment,the display device 100 uses a liquid crystal (LC) display panel 102,which typically comprises a layer of LC 104 disposed between panelplates 106. The plates 106 are often formed of glass, or another stiffmaterial, and may include electrode structures and alignment layers ontheir inner surfaces for controlling the orientation of the liquidcrystals in the LC layer 104. The electrode structures are commonlyarranged so as to define LC panel pixels, areas of the LC layer wherethe orientation of the liquid crystals can be controlled independentlyof adjacent pixels. A color filter may also be included with one or moreof the plates 106 for imposing color on the displayed image.

An upper absorbing polarizer 108 is positioned above the LC layer 104and a lower absorbing polarizer 110 is positioned below the LC layer104. In the illustrated embodiment, the upper and lower absorbingpolarizers 108, 110 are located outside the LC panel 102. The absorbingpolarizers 108, 110 and the LC panel 102, in combination, control thetransmission of light from a backlight 112 through the display 100 tothe viewer. In some exemplary embodiments, when a pixel of the LC layer104 is not activated, it does not change the polarization of lightpassing therethrough. Accordingly, light that passes through the lowerabsorbing polarizer 110 is absorbed by the upper absorbing polarizer108, when the absorbing polarizers 108, 110 are aligned perpendicularly.When the pixel is activated, on the other hand, the polarization of thelight passing therethrough is rotated, so that at least some of thelight that is transmitted through the lower absorbing polarizer 110 isalso transmitted through the upper absorbing polarizer 108. Selectiveactivation of the different pixels of the LC layer 104, for example by acontroller 113, results in the light passing out of the display atcertain desired locations, thus forming an image seen by the viewer. Thecontroller 113 may include, for example, a computer or a televisioncontroller that receives and displays television images.

One or more optional layers 109 may be provided over the upper absorbingpolarizer 108, for example to provide mechanical and/or environmentalprotection to the display surface. In one exemplary embodiment, thelayer 109 may include a hardcoat over the absorbing polarizer 108.

Some types of LC displays may operate in a manner different from thatdescribed above and, therefore, differ in detail from the describedsystem. For example, the absorbing polarizers may be aligned paralleland the LC panel may rotate the polarization of the light when in anunactivated state. Regardless, the basic structure of such displaysremains similar to that described above.

The backlight 112 comprises one or more illumination light units 114that generate the illumination light and direct the illumination lightinto a lightguide 120. The illumination light units 114 include a numberof light sources 116 to generate the illumination light. The lightsources 116 may be extended light sources that emit light over anextended length. One example of an extended light source is a coldcathode, fluorescent tube. The light sources 116 may also be effectivepoint light sources, for example, light emitting diodes (LEDs). Othertypes of light sources may also be used, such as organic LEDs (OLEDs).This list of light sources is not intended to be limiting or exhaustive.

The light sources 116 may be operated within a light reflecting cavity118 that is used to collect and direct light to the lightguide 120. Thelightguide 120 guides illumination light from the light sources 116 toan area behind the display panel 102, and directs the light to thedisplay panel 102. The light guide 120 may receive illumination lightthrough a single edge, or through multiple edges. In other embodiments,not illustrated, the light may be coupled into the light guide 120through a light coupling mechanism other than the edge of the lightguide 120.

A base reflector 122 may be positioned on the other side of the lightguide 120 from the display panel 102. The light guide 120 may includelight extraction features 123 that are used to extract the light fromthe lightguide 120 for illuminating the display panel 102. For example,the light extraction features 123 may comprise diffusing spots on thesurface of the light guide 120 that direct light either directly towardsthe display panel 102 or towards the base reflector 122. Otherapproaches may be used to extract the light from the light guide 120.

The base reflector 122 may also be useful for recycling light within thedisplay device 100, as is explained below. The base reflector 122 may bea specular reflector or may be a diffuse reflector. One example of aspecular reflector that may be used as the base reflector 122 isVikuiti™ Enhanced Specular Reflection (ESR) film available from 3MCompany, St. Paul, Minn. Examples of suitable diffuse reflectors includepolymers, such as polyethylene terephthalate (PET), polycarbonate (PC),polypropylene, polystyrene and the like, loaded with diffuselyreflective particles, such as titanium dioxide, barium sulphate, calciumcarbonate and the like. Other examples of diffuse reflectors, includingmicroporous materials and fibril-containing materials, are discussed inco-owned U.S. Patent Application Publication 2003/0118805 A1,incorporated herein by reference.

An arrangement of light management layers 124 may be positioned betweenthe backlight 112 and the display panel 102 for enhanced performance.For example, the light management layers 124 may include a reflectivepolarizer 126. The light sources 116 typically produce unpolarized lightbut the lower absorbing polarizer 110 only transmits a singlepolarization state, and so about half of the light generated by thelight sources 116 is not suitable for transmission through to the LClayer 104. The reflecting polarizer 126, however, may be used to reflectthe light that would otherwise be absorbed in the lower absorbingpolarizer 110, and so this light may be recycled by reflection betweenthe reflecting polarizer 126 and the base reflector 122. At least someof the light reflected by the reflecting polarizer 126 may bedepolarized and subsequently returned to the reflecting polarizer 126 ina polarization state that is transmitted through the reflectingpolarizer 126 and the lower absorbing polarizer 110 to the LC panel 102.In this manner, the reflecting polarizer 126 may be used to increase thefraction of light emitted by the light sources 116 that reaches the LCpanel 102, and so the image produced by the display device 100 isbrighter.

Any suitable type of reflective polarizer may be used, for example,multilayer optical film (MOF) reflective polarizers, diffuselyreflective polarizing film (DRPF), such as continuous/disperse phasepolarizers, wire grid reflective polarizers or cholesteric reflectivepolarizers.

Both the MOF and continuous/disperse phase reflective polarizers rely onthe difference in refractive index between at least two materials,usually polymeric materials, to selectively reflect light of onepolarization state while transmitting light in an orthogonalpolarization state. Some examples of MOF reflective polarizers aredescribed in co-owned U.S. Pat. Nos. 5,882,774, incorporated herein byreference. Commercially available examples of MOF reflective polarizersinclude Vikuiti™ DBEF-D200 and DBEF-D400 multilayer reflectivepolarizers that include diffusive surfaces, available from 3M Company,St. Paul, Minn.

Examples of DRPF useful in connection with the present invention includecontinuous/disperse phase reflective polarizers as described in co-ownedU.S. Pat. No. 5,825,543, incorporated herein by reference, and diffuselyreflecting multilayer polarizers as described in e.g. co-owned U.S. Pat.No. 5,867,316, also incorporated herein by reference. Other suitabletypes of DRPF are described in U.S. Pat. No. 5,751,388.

Some examples of wire grid polarizers useful in connection with thepresent invention include those described in U.S. Pat. No. 6,122,103.Wire grid polarizers are commercially available from, inter alia, MoxtekInc., Orem, Utah.

Some examples of cholesteric polarizers useful in connection with thepresent invention include those described in, for example, U.S. Pat. No.5,793,456, and U.S. Patent Publication No. 2002/0159019. Cholestericpolarizers are often provided along with a quarter wave retarding layeron the output side, so that the light transmitted through thecholesteric polarizer is converted to linear polarization.

A polarization mixing layer 128 may be placed between the backlight 112and the reflecting polarizer 126 to aid in mixing the polarization ofthe light reflected by the reflecting polarizer 126. For example, thepolarization mixing layer 128 may be a birefringent layer such as aquarter-wave retarding layer.

The light management layers 124 may also include one or more prismaticbrightness enhancing layers 130 a, 130 b. A prismatic brightnessenhancing layer is one that includes a surface structure that redirectsoff-axis light into a propagation direction closer to the axis 132 ofthe display device 100. This controls the viewing angle of theillumination light passing through the display panel 102, typicallyincreasing the amount of light propagating on-axis through the displaypanel 102. Consequently, the on-axis brightness of the image seen by theviewer is increased.

One example of a brightness enhancing layer has a number of prismaticridges that redirect the illumination light through a combination ofrefraction and reflection. Examples of prismatic brightness enhancinglayers that may be used in the display device include the Vikuiti™ BEFIIand BEFIII family of prismatic films available from 3M Company, St.Paul, Minn., including BEFII 90/24, BEFII 90/50, BEFIIIM 90/50, andBEFIIIT. Although only one brightness enhancing layer may be used, twobrightness enhancing layers 130 a, 130 b may be used, with theirstructures oriented at about 90° to each other. This crossedconfiguration provides control of the viewing angle of the illuminationlight in two dimensions, the horizontal and vertical viewing angles.

One exemplary embodiment of the illumination light unit 199 is nowdescribed with reference to FIG. 2A. The figure shows part of theillumination light unit 199, including some light sources 116 a, 116 b.A reflecting cavity 118 may be formed between at least one reflectingsurface 202 and a controlled transmission mirror 200 that are arrangedso that at least some of the illumination light produced by the sources116 a, 116 b is reflected by both the controlled transmission mirror 200and the reflecting surface 202. In the illustrated embodiment, thereflecting surface 202 is positioned behind the light sources 116 a, 116b. The reflecting cavity 118 advantageously provides uniform edgeillumination for back-lit displays that use linear light sources, suchas CCFLs, or quasi-point light sources, such as LEDs, but may also beused with other types of light sources.

The controlled transmission mirror 200 comprises a multilayer reflector204 that has a reflection spectrum such that at least some of the lightgenerated by the light sources 116 a, 116 b, when normally incident onthe multilayer reflector 204, is reflected.

An input coupling element 206 is disposed at the lower side of themultilayer reflector 204, and an output coupling element 208 is disposedat the upper side of the multilayer reflector 204. The input couplingelement 206 and output coupling element 208 are used to change directionof at least some of the light entering these coupling elements 206, 208,so as to couple light through the controlled transmission mirror 200.Exemplary embodiments of input coupling elements 206 and output couplingelements 208 include diffusers, both surface and bulk diffusers, andmicroreplicated surfaces. Examples of suitable input coupling elements206 and output coupling elements 208 are described in greater detailbelow. The output coupling element 208 may be the same type of couplingelement as the input coupling element 206, for example, the input andoutput coupling element 206, 208 may both be bulk diffusers, or may bedifferent from the input coupling element 206. The input and outputcoupling elements 206, 208 may be laminated or otherwise formedintegrally with the multilayer reflector 204.

The multilayer dielectric reflector 204 is generally constructed ofoptical repeating units that form the basic building blocks of adielectric stack. The optical repeating units typically include two ormore layers of at least a high and a low refractive index material. Amultilayer reflector can be designed, using these building blocks, toreflect infrared, visible or ultraviolet wavelengths and one or both ofa given orthogonal pair of polarizations of light. In general, the stackcan be constructed to reflect light of a particular wavelength, λ, bycontrolling the optical thickness of the layers according to therelationship:

λ=(2/M)*D _(r),   (1)

where M is an integer representing the order of the reflected light, andD_(r) is the optical thickness of an optical repeating unit. For thefirst order reflection (M=1), the optical repeating unit has an opticalthickness of λ/2. Simple quarter-wave stacks comprise a number of layersthat each have an optical thickness of one quarter of the wavelength,λ/4. Broadband reflectors can include multiple quarter-wave stacks tunedto various wavelengths, a stack with a continuous gradation of the layerthickness throughout the stack, or combinations thereof. A multilayerreflector may further include non-optical layers. For example, acoextruded polymeric dielectric reflector may include protectiveboundary layers and/or skin layers used to facilitate formation of thereflector film and to protect the reflector. Polymeric optical stacksparticularly suited to the present invention are described in publishedPCT Patent Application WO 95/17303, entitled MULTILAYER OPTICAL FILM,and U.S. Pat. No. 6,531,230, incorporated herein by reference. In otherembodiments, the dielectric stack may be a stack of inorganic materials.Some suitable materials used for the low refractive index materialinclude SiO₂, MgF₂ and CaF₂ and the like. Some suitable materials usedfor the high refractive index material include TiO₂, Ta₂O₅, ZnSe and thelike. The invention is not limited to quarter-wave stacks, however, andis more generally applicable to any dielectric stack including, forexample, computer optimized stacks and random layer thickness stacks.

Reflection by a dielectric stack of light at a particular wavelength isdependent, in part, on the propagation angle through the stack. Themultilayer reflector 204 may be considered as having a reflection bandprofile (e.g., band center and bandedges) for light propagating in thestack at a particular angle. This band profile changes as the angle ofpropagation in the stack changes. The propagation angle in the stack isgenerally a function of the incident angle and the refractive indices ofthe materials in the stack and the surrounding medium. The wavelength ofthe reflection bandedge changes as the propagation angle in the stackchanges. Typically, for the polymeric materials under consideration, thebandedge of the reflector for light at normal incidence shifts to about80% of its normal incidence value when viewed at grazing incidence inair. This effect is described in greater detail in U.S. Pat. No.6,208,466, incorporated herein by reference. The bandedge may shiftconsiderably further when the light is coupled into the reflector usinga medium having a refractive index higher than air. Also, the shift inthe bandedge is typically greater for p-polarization light than fors-polarization light.

The angular dependence of the reflection band profile, e.g., bandedgeshifting with angle, results from a change in the effective layerthickness. The reflection band shifts towards shorter wavelengths as theangle increases from normal incidence. While the total path lengththrough a given layer increases with angle, the change in band positionwith angle does not depend on the change in the total path lengththrough a layer with angle, θ, where the angle is measured relative toan axis 230 perpendicular to the layers of the reflector 204. Rather,the band position depends on the difference in path length between lightrays reflected from the top and bottom surfaces of a given layer. Thispath difference decreases with angle of incidence as shown by thefamiliar formula n.d.cosθ, which is used to calculate the wavelength, λ,to which a given layer is tuned as a λ/4 thick layer.

The above description describes how the bandedge of the reflection bandprofile changes as a function of angle. As used herein, the termbandedge generally refers to the region where the multilayer reflectorchanges from substantial reflection to substantial transmission. Thisregion may be fairly sharp and described as a single wavelength. Inother cases, the transition between reflection and transmission may bemore gradual and may be described in terms of a center wavelength andbandwidth. In either case, however, a substantial difference betweenreflection and transmission exists on either side of the bandedge.

As light at the particular wavelength propagates in the stack atincreasing propagation angles (measured from an axis normal to theinterface of the repeating units), the light approaches the bandedge. Inone example, at high enough propagation angles, the stack will becomesubstantially transparent to that particular wavelength of light and thelight will transmit through the stack. Thus, for a given wavelength oflight, the stack has an associated propagation angle below which thestack substantially reflects the light and another propagation angleabove which the stack substantially transmits the light. Accordingly, incertain multilayer stacks, each wavelength of light may be considered ashaving a corresponding angle below which substantial reflection occurs,and a corresponding angle above which substantial transmission occurs.The sharper the bandedge, the closer these two angles are for theassociated wavelength. For the purposes of the present description, theapproximation is made that these two angles are the same and have avalue of θ_(min).

The above description describes the manner in which monochromatic lightin a given stack shifts from reflection to transmission with increasingangle of propagation. If the stack is illuminated with light having amixture of components at different wavelengths, the angle, θ_(min) atwhich the reflective stack changes from being reflective to transmissiveis likely to be different for the different wavelength components. Sincethe bandedge moves to shorter wavelengths with increasing angle, thevalue of θ_(min) is lower for light at longer wavelengths, potentiallyallowing the more light at longer wavelengths to be transmitted throughthe multilayer reflector than at shorter wavelengths. In someembodiments it is desired that the color of the light passing out of thecontrolled transmission mirror be relatively uniform. One approach tobalancing the color is to use an input and output coupling element thatcouples more light at shorter wavelengths than at longer wavelengthsinto the controlled transmission mirror.

One example of such a coupling element is a bulk diffuser that containsscattering particles dispersed within a polymer matrix, as is discussedbelow with regards to FIGS. 3A and 4A. The scattering particles have arefractive index different from the surrounding matrix. The nature ofdiffusive scattering is that, all else being equal, light at shorterwavelengths is scattered more than light at longer wavelengths.

In addition, the degree of scattering is dependent on the differencebetween the refractive indices of the particles and the surroundingmatrix. If the difference in refractive index is greater at shorterwavelengths, then even more short wavelength light is scattered. In oneparticular embodiment of a diffusive coupling element, the matrix isformed of biaxially stretched PEN, which has an in-plane refractiveindex of about 1.75 for red light and about 1.85 for blue light, wherethe light is s-polarized, i.e., has high dispersion. The in-planerefractive index is the refractive index for light whose electric vectoris polarized parallel to the plane of the film. The out-of-planerefractive index, for light polarized parallel to the thicknessdirection of the film, is about 1.5. The refractive index forp-polarized light is lower than that of the s-polarized light, since thep-polarized light experiences an effective refractive index that is acombination of the in-plane refractive index and the out-of-planerefractive index. The particles in the matrix may have a high refractiveindex, for example titanium dioxide (TiO₂) particles have a refractiveindex of about 2.5. The refractive index of TiO₂ varies by approximately0.25 over the range 450 nm-650 nm, which is greater than theapproximately 0.1 refractive index variation for PEN over a similarwavelength range. Thus, the refractive index difference between theparticles and the matrix changes by about 0.15 across the visiblespectrum, resulting in increased scattering for the blue light.Consequently, the refractive index difference between the particles andthe matrix can vary significantly over the visible spectrum.

Thus, due to the wavelength dependence of the diffusive scatteringmechanism and the large difference in the refractive index differenceover the visible spectrum, the degree to which blue light is scatteredinto the multilayer reflector is relatively high, which at leastpartially compensates for the larger value of θ_(min) at shorterwavelengths.

Other embodiments of input and output coupling elements, for examplethose described below with reference to FIGS. 3B-3D and 4B-4D, relyprimarily on refractive effects for diverting the light. For example, acoupling element may be provided with a surface structure or holographicfeatures for coupling the light into or out of the multilayer reflector.Normal material dispersion results in greater refractive effects forshorter wavelengths. Therefore, input and output coupling elements thatrely on refractive effects may also at least partially compensate forthe larger value of θ_(min) at shorter wavelengths.

With the understanding, therefore, that the light entering thecontrolled transmission mirror may have a wide variation in the value ofθ_(min), the following description refers to only a single value ofθ_(min) for simplicity.

Another effect that the system designer can use to control the amount oflight passing through the multilayer reflector is the selection of aBrewster's angle, i.e., the angle at which p-polarized light passesthrough the multilayer reflector without reflective loss. For adjacentisotropic layers 1 and 2 in the multilayer reflector, having refractiveindices n1 and n2 respectively, the value of Brewster's angle in layer1, θ_(B), for light passing from layer 1 to layer 2, is given by theexpression tan θ_(B)=n2/n1. Thus, the particular materials employed inthe different layers of the multilayer reflector may be selected toprovide a desired value of Brewster's angle.

The existence of the Brewster's angle for a multilayer reflectorprovides another mechanism for allowing light to pass through thereflector other than relying on the input and output coupling layers todivert the light through large angles. As the angle within thecontrolled transmission mirror is increased for p-polarized light, thereflection band substantially disappears at Brewster's angle. At anglesabove the Brewster's angle, the reflection band reappears and continuesto shift to shorter wavelengths.

In certain embodiments, it may be possible to set the value of θ_(B) forblue light to be less than θ_(min), but have θ_(B) be greater thanθ_(min) for red light. This configuration may lead to an increasedtransmission for blue light through the multilayer reflector, whichcompensates at least in part for the higher value of θ_(min) for shorterwavelength light.

Returning to FIG. 2A, at least some of the light from the light source116 a propagates towards the controlled transmission mirror 200. Aportion of the light, exemplified by light ray 210, passes through theinput coupling element 206 and is incident on the multilayer reflector204 at an angle greater than θ_(min) and is transmitted through thereflector 204. Another portion of the light, exemplified by light ray212, is incident at the input coupling element 206 at an angle less thanθ_(min), but is diverted by the input coupling element 206 to an angleof at least θ_(min), and is transmitted through the multilayer reflector204. Another portion of light from the light source 116 a, exemplifiedby light ray 214, passes through the input coupling element 206 and isincident at the multilayer reflector 204 at an angle that is less thanθ_(min). Consequently, light 214 is reflected by the multilayerreflector 204 to the reflecting surface 202. The light 214 may bereflected at the reflecting surface 202 either specularly or diffusely.

In some embodiments it may be desired that the multilayer reflector 204is attached to the output coupling element 208 in a manner that avoids alayer of air, or some other material of a relatively low refractiveindex, between the multilayer reflector 204 and output coupling element208. Such close optical coupling between the multilayer reflector 204and the output coupling element 208 reduces the possibility of totalinternal reflection of light at the multilayer reflector 204.

The maximum angle of the light within the controlled transmission mirror200, θ_(max), is determined by the relative refractive indices of theinput coupling element 206, n_(i), and the refractive index of theparticular layer of the multilayer reflector 204, n₁, n₂, where thesubscripts 1, 2 refer to the alternating layers in the multilayerreflector 204. Where the input coupling element 206 is a surfacecoupling element, the value of n_(i) is equal to the refractive index ofthe material on which the coupling surface is formed. The effectiverefractive index of the multilayer reflector 204 is the average of therefractive indices of the high index and low index layers. Propagationfrom the input coupling element 206 into multilayer reflector 204 issubject to Snell's law. The value of θ_(max) in each alternate layer ofthe multilayer reflector 204 is given by the expression:

θ_(max)=sin⁻¹ (n_(i)/n_(1,2)).   (2)

where either n₁ or n₂ is used. Where n_(i)>n₁ and n_(i)>n₂, then θ_(max)can be up to 90°.

The output coupling element 208 is used to extract at least some of thelight out of the controlled transmission mirror 200. For example, someof light 212 may be diffused by the output coupling element 208 so as topass out of the controlled transmission mirror 200 as light 220.

Other portions of the light, for example ray 222, may not be diverted bythe output coupling element 208. If light 222 is incident at the uppersurface of the output coupling element 208 at an angle greater than thecritical angle of the output coupling element, θ_(c)=sin⁻¹ (1/n₀), wheren₀ is the refractive index of the output coupling element, then thelight 222 is totally internally reflected within the output couplingelement 208 as light 224. The reflected light 224 may subsequently betotally internally reflected at the lower surface of the input couplingelement 206. Alternatively, the light 224 may subsequently be divertedby the input coupling element 206 and pass out of the controlledtransmission mirror 200 towards the reflecting surface 202.

If the light that passes into the multilayer reflector 204 with an angleof at least θ_(min) is incident at the output coupling element 208 withan angle greater than θ_(c), then that light which is not diverted outof the output coupling element 208 is typically totally internallyreflected within the output coupling element 208. If, however, the lightthat passes into the multilayer reflector 204 with an angle of at leastθ_(min) reaches the output coupling element 208 at a propagation angleless than θ_(c) then a fraction of that light may be transmitted outthrough the output coupling element 208, even without being diverted bythe output coupling element 208, subject to Fresnel reflection loss atthe interface between the output coupling element 208 and the air. Thus,there are many possibilities for the light to suffer multiplereflections and for its direction to be diverted within the cavity 114.The light may also propagate transversely within the substrate 202and/or within the space between the controlled transmission mirror 120and the base reflector 118. These multiple effects combine to increasethe likelihood that the light is spread laterally and extracted toproduce a backlight illuminance of uniform brightness.

Except for the possibility that the multilayer reflector has a value ofBrewster's angle, θ_(B), that is lower than θ_(min), there is aforbidden angular region for light originating at the light source 116a. This forbidden angular region has a half-angle of θ_(min), and islocated above the light source 116 a. Light cannot pass through themultilayer reflector 204 within the forbidden angular region. Light 232from a neighboring light source 116 b, however, may be able to escapefrom the controlled transmission mirror 200 at a point perpendicularlyabove light source 116 a, at the axis 230, and so the illumination lightunit 199 is effective at mixing light from different light sources 116a, 116 b.

In view of the description of the controlled transmission mirror 200provided above, it can be seen that the function of the input couplingelement 206 is to divert at least some light, that would otherwise beincident at the multilayer reflector 204 at an angle less than θ_(min),so as to be incident at the multilayer reflector 204 at an angle of atleast θ_(min). Also, the function of the output coupling element 208 isto divert at least some light, that would otherwise be totallyinternally reflected within the multilayer reflector 204, so as to passout of the controlled transmission mirror 200.

The controlled transmission mirror 200 may include a transparent layer250 disposed between the output coupling element 208 and the multilayerreflector 204, as is schematically illustrated in FIG. 2B. In otherembodiments, the transparent layer 250 may be between the input couplingelement 206 and the multilayer reflector 204. The transparent layer 250may be formed of any suitable transparent material, organic orinorganic, for example polymer or glass. Suitable polymer materials maybe amorphous or semi-crystalline, and may include homopolymer, copolymeror blends thereof. Example polymer materials include, but are notlimited to, amorphous polymers such as poly(carbonate) (PC);poly(styrene) (PS); acrylates, for example acrylic sheets as suppliedunder the ACRYLITE® brand by Cyro Industries, Rockaway, N.J.; acryliccopolymers such as isooctyl acrylate/acrylic acid;poly(methylmethacrylate) (PMMA); PMMA copolymers; cycloolefins;cylcoolefin copolymers; acrylonitrile butadiene styrene (ABS); styreneacrylonitrile copolymers (SAN); epoxies; poly(vinylcyclohexane);PMMA/poly(vinylfluoride) blends; atactic poly(propylene); poly(phenyleneoxide) alloys; styrenic block copolymers; polyimide; polysulfone;poly(vinyl chloride); poly(dimethyl siloxane) (PDMS); polyurethanes; andsemicrystalline polymers such as poly(ethylene); poly(propylene);poly(ethylene terephthalate) (PET); poly(carbonate)/aliphatic PETblends; poly(ethylene naphthalate)(PEN); polyamides; ionomers; vinylacetate/polyethylene copolymers; cellulose acetate; cellulose acetatebutyrate; fluoropolymers; poly(styrene)-poly(ethylene) copolymers; PETand PEN copolymers, and clear fiberglass panels. Some of thesematerials, for example PET, PEN and copolymers thereof, may be orientedso as to change the material refractive index from that of the isotropicmaterial.

The transparent layer 250 may be used to allow more lateral spreading ofthe light from the light sources 116 before extracting the light fromthe controlled transmission mirror 200 using the output coupling element208.

One or more of the edges of the transparent layer 250 may be covered bya reflector 252. Thus, light 254 that might otherwise escape from thetransparent layer 250 is reflected back into the transparent layer 250and may be extracted from the illumination light unit 114 as usefulillumination light. The reflector 254 may be any suitable type ofreflector, including a multilayer dielectric reflector, a metal coatingon the edge of the transparent layer 250, a multilayer polymerreflector, a diffuse polymer reflector, or the like. In the illustratedembodiment, the reflector 252 at the side of the lower reflector 202 maybe also used as a side reflector for the reflecting cavity 118, althoughthis is not intended to be a limitation of the invention.

In some other embodiments, the controlled transmission mirror 200 may beprovided with two multilayer reflectors positioned on either side of thetransparent layer 250. The multilayer reflectors may have the same valueof θ_(min), although this is not required. The use of a transparentlayer is described further in U.S. patent application Ser. No. ______,titled “OPTICAL ELEMENT FOR LATERAL LIGHT SPREADING IN BACK-LIT DISPLAYSAND SYSTEM USING SAME” filed on even date herewith, and having attorneydocket no. 60499US002, incorporated herein by reference.

Exemplary embodiments of different types of input coupling elements arenow discussed with reference to FIGS. 3A-3D. In other exemplaryembodiments, not illustrated, a transparent layer may be providedbetween the multilayer reflector and either of the input and outputcoupling elements.

In FIG. 3A, an exemplary embodiment of a controlled transmission mirror320 comprises an input coupling element 326, a multilayer reflector 304and an output coupling element 308. In this particular embodiment, theinput coupling element 326 is a bulk diffusing layer, comprisingdiffusing particles 326 a dispersed within a transparent matrix 326 b.At least some of the light entering the input coupling element 326 at anangle less than θ_(min), for example light rays 328, is scattered withinthe input coupling element 326 at an angle greater than θ_(min), and isconsequently transmitted through the multilayer reflector 304. Somelight, for example ray 330, may not be scattered within the inputcoupling element 326 through a sufficient angle to pass through themultilayer reflector 304, and is reflected by the multilayer reflector304. Suitable materials for the transparent matrix 326 b include, butare not limited to, polymers such as those listed as being suitable foruse in a transparent layer above.

The diffusing particles 326 a may be any type of particle useful fordiffusing light, for example, transparent particles whose refractiveindex is different from the surrounding polymer matrix, diffuselyreflective particles, or voids or bubbles in the matrix 326 b. Examplesof suitable transparent particles include solid or hollow inorganicparticles, for example glass beads or glass shells, solid or hollowpolymeric particles, for example solid polymeric spheres or polymerichollow shells. Examples of suitable diffusely reflecting particlesinclude particles of titanium dioxide (TiO₂), calcium carbonate (CaCO₃),barium sulphate (BaSO₄), magnesium sulphate (MgSO₄) and the like. Inaddition, voids in the matrix 426 b may be used for diffusing the light.Such voids may be filled with a gas, for example air or carbon dioxide.

Another exemplary embodiment of a controlled transmission mirror 340 isschematically illustrated in FIG. 3B, in which the input couplingelement 346 comprises a surface diffuser 346 a. The surface diffuser 346a may be provided on the bottom layer of the multilayer reflector 304 oron a separate layer attached to the multilayer reflector 304. Thesurface diffuser 346 a may be molded, impressed, cast or otherwiseprepared.

At least some of the light incident at the input coupling element 346,for example light rays 348, is scattered by the surface diffuser 346 ato propagate at an angle greater than θ_(min), and is consequentlytransmitted through the multilayer reflector 304. Some light, forexample ray 350, may not be scattered by the surface diffuser 346 athrough a sufficient angle to pass through the multilayer reflector 304,and is reflected.

Another exemplary embodiment of a controlled transmission mirror unit360 is schematically illustrated in FIG. 3C, in which the input couplingelement 366 comprises a microreplicated structure 367 having facets 367a and 367 b. The structure 367 may be provided on the bottom layer ofthe multilayer reflector 304 or on a separate layer attached to themultilayer reflector 304. The structure 367 is different from thesurface diffuser 346 a of FIG. 3B in that the surface diffuser 346 aincludes a mostly random surface structure, whereas the structure 367includes more regular structures with defined facets 367 a, 367 b.

At least some of the light incident at the input coupling element 366,for example rays 368 incident on facets 367 a, would not reach themultilayer reflector 304 at an angle of θ_(min) but for refraction atthe facet 367 a. Accordingly, light rays 368 are transmitted through themultilayer reflector 304. Some light, for example ray 370, may berefracted by facet 367 b to an angle less than θ_(min), and is,therefore, reflected by the multilayer reflector 304.

Another exemplary embodiment of a controlled transmission mirror 380 isschematically illustrated in FIG. 3D, in which the input couplingelement 386 has surface portions 382 in optical contact with themultilayer reflector 304 and other surface portions 384 that do not makeoptical contact with the multilayer reflector 304, with a gap 388 beingformed between the element 386 and the multilayer reflector 304. Thepresence of the gap 388 provides for total internal reflection (TIR) ofsome of the incident light. This type of element may be referred to as aTIR input coupling element.

At least some of the light incident at the input coupling element 386,for example rays 390 incident on the non-contacting surface portions 384would not reach the multilayer reflector 304 at an angle of θ_(min) butfor internal reflection at the surface 384. Accordingly, light rays 390may be transmitted through the multilayer reflector 304. Some light, forexample ray 392, may be transmitted through the contacting surfaceportion 382 to the multilayer reflector 304. This light is incident atthe multilayer reflector 302 at an angle less than θ_(min), and so isreflected by the multilayer reflector 304.

Other types of TIR input coupling elements are described in greaterdetail in U.S. Pat. No. 5,995,690, incorporated herein by reference.

Other types of input coupling elements may be used in addition to thosedescribed in detail here, for example input coupling elements thatinclude a surface or a volume hologram. Also, an input coupling elementmay combine different approaches for diverting light. For example, aninput coupling element may combine a surface treatment, such as asurface structure or surface scattering pattern, or surface hologram,with bulk diffusing particles.

It may be desired in some embodiments for the refractive index of theinput coupling element and output coupling element to have a relativelyhigh refractive index, for example comparable to or higher than theeffective refractive index (the average of the refractive indices of thehigh index and low index layers) of the multilayer reflector 304. Ahigher refractive index for the input and output coupling elements helpsto increase the angle at which light may propagate through themultilayer reflector 304, which leads to a greater bandedge shift. This,in turn, may increase the amount of short wavelength light that passesthrough the controlled transmission mirror, thus making the color of thebacklight illumination more uniform. Examples of suitable highrefractive index polymer materials that may be used for input and outputcoupling elements include biaxially stretched PEN and PET whichdepending on the amount of stretch, can have in-plane refractive indexvalues of 1.75 and 1.65 respectively for a wavelength of 633 nm.

Commensurate with the choice of materials for the input and outputcoupling elements, the substrate should be chosen to have an index thatdoes not cause TIR that would block prohibitive amounts of lightentering or exiting at large angles. Conversely, a low index for thesubstrate would result in high angles of propagation in the substrateafter injection from the input coupler having a higher index than thesubstrate. These two effects can be chosen to optimize the performanceof the system with respect to color balance and lateral spreading of thelight.

Similar approaches may be used for the output coupling element. Forexample, a controlled transmission mirror unit 420 is schematicallyillustrated in FIG. 4A having an input coupling element 406, amultilayer reflector 404 and an output coupling element 428. In thisparticular embodiment, the output coupling element 428 is a bulkdiffusing layer, comprising diffusing particles 428 a dispersed within atransparent matrix 428 b. Suitable materials for use as the diffusingparticles 428 a and the matrix 428 b are discussed above with respect tothe input coupling element 326 of FIG. 3A.

At least some of the light entering the output coupling element 428 fromthe multilayer reflector 404, for example light ray 430, may bescattered by the diffusing particles 428 a in the output couplingelement 408 and is consequently transmitted out of the light outputcoupling element 428. Some light, for example ray 432, may not bescattered within the output coupling element 428 and is incident at thetop surface 429 of the output coupling element 428 at an incident angleof θ. If the value of θ is equal to or greater than the critical angle,θ_(c), for the material of the matrix 428 b, then the light 432 istotally internally reflected at the surface 429.

Another exemplary embodiment of controlled transmission reflector 440 isschematically illustrated in FIG. 4B, in which the output couplingelement 448 comprises a surface diffuser 448 a. The surface diffuser 448a may be provided on the upper surface of the multilayer reflector 404or on a separate layer attached to the multilayer reflector 404.

Some light propagating within the multilayer reflector 404, for examplelight 450, is incident at the surface diffuser 448 a and is scatteredout of the light mixing layer 440. Some other light, for example light452, may not be scattered by the surface diffuser 448 a. Depending onthe angle of incidence at the surface diffuser 448 a, the light 452 maybe totally internally reflected, as illustrated, or some light may betransmitted out of the controlled transmission mirror 440 while some isreflected back within the multilayer reflector 404.

Another exemplary embodiment of controlled transmission mirror 460 isschematically illustrated in FIG. 4C, in which the output couplingelement 466 comprises a microreplicated structure 467 having facets 467a and 467 b. The structure 467 may be provided on a separate layer 468attached to the multilayer reflector 404, as illustrated, or be integralwith the top surface of the multilayer reflector 404 itself. Thestructure 467 is different from the surface diffuser 448 a of FIG. 4B inthat the surface diffuser 448 a includes a mostly random surfacestructure, whereas the structure 467 includes more regular structureswith defined facets 467 a, 467 b.

Some light propagating within the multilayer reflector 404, for examplelight 470, is incident at the surface diffuser structure 467 and isrefracted out of the light mixing layer 460. Some other light, forexample light 472, may not be refracted out of the light mixing layer460 by the structure 467, but may be returned to the multilayerreflector 404. The particular range of propagation angles for light toescape from the controlled transmission mirror 460 is dependent on anumber of factors, including at least the refractive indices of thedifferent layers that make up the controlled transmission mirror 460 andthe layer 468 as well as the shape of the structure 467.

Another exemplary embodiment of a controlled transmission mirror 480 isschematically illustrated in FIG. 4D, in which the output couplingelement 486 comprises a light coupling tape that has surface portions482 in optical contact with the multilayer reflector 404 and othersurface portions 484 that do not make optical contact with themultilayer reflector 404, forming a gap 488 between the element 486 andthe multilayer reflector 404.

At least some of the light incident at the output coupling element 486,for example light ray 490, is incident at a portion of the multilayerreflector's surface that is not contacted to the output couplingelement, but is adjacent to a gap 488, and so the light 490 is totallyinternally reflected within the multilayer reflector 404. Some light,for example ray 492, may be transmitted through the contacting surfaceportion 482, and be totally internally reflected at the non-contactingsurface portion 484, and so is coupled out of the controlledtransmission mirror 480.

Other types of output coupling elements may be used in addition to thosedescribed in detail here. Also, an output coupling element may combinedifferent approaches for diverting light out of the controlledtransmission mirror. For example, an output coupling element may combinea surface treatment, such as a surface structure or surface scatteringpattern, with bulk diffusing particles.

In some embodiments, the output coupling element may be constructed sothat the degree to which light is extracted is uniform across the outputcoupling element. In other embodiments, the output coupling element maybe constructed so that the degree to which light is extracted out of thecontrolled transmission mirror is not uniform across the output couplingelement. For example, in the embodiment of output coupling element 428illustrated in FIG. 4A, the density of diffusing particles 428 a may bevaried across the output coupling element 428 so that a higher fractionof light can be extracted from some portions of the output couplingelement 428 than others. In the illustrated embodiment, the density ofdiffusing particles 428 a is higher at the left side of the outputcoupling element 428. Likewise, for the embodiments of controlledtransmission mirrors 440 460, 480 illustrated in FIGS. 4B-4D, the outputcoupling elements 448, 468, 488 may be shaped or designed so that ahigher fraction of light can be extracted from some portions of theoutput coupling elements 448, 468, 488 than from other portions. Theprovision of non-uniformity in the extraction of the light from thecontrolled transmission mirror, for example extracting a smallerfraction of light from portions of the controlled transmission mirrorthat contain more light and extracting a higher fraction of light fromportions of the controlled transmission mirror that contain less light,may result in a more uniform brightness profile in the illuminationlight propagating towards the display panel.

The number of bounces made by light within the controlled transmissionmirror, and therefore, the uniformity of the extracted light, may beaffected by the reflectivity of both the input coupling element and theoutput coupling element. The trade-off for uniformity is brightness losscaused by absorption in the input coupling element, the multilayerreflector and the output coupling element. This absorption loss may bereduced by proper choice of materials and material processingconditions.

In some exemplary embodiments, the controlled transmission mirror may bepolarization sensitive, so that light in one polarization state ispreferentially extracted. A cross-section through one exemplaryembodiment of a polarization sensitive controlled transmission mirror520 is schematically illustrated in FIG. 5A. The controlled transmissionmirror 520 comprises an optional transparent layer 502, a multilayerreflector 504, an input coupling element 506 and a polarizationsensitive output coupling element 528. A three-dimensional coordinatesystem is used here to clarify the following description. The axes ofthe coordinate system have been arbitrarily assigned so that the planeof the controlled transmission mirror 520 lies parallel to the x-yplane, with the z-axis having a direction through the thickness of thecontrolled transmission mirror 520. The lateral dimension shown in FIG.5A is parallel to the x-axis, and the y-direction extends in a directionperpendicular to the drawing.

In some embodiments, the extraction of only one polarization of thelight propagating within the controlled transmission mirror 520 iseffected by the output coupling element 528 containing two materials,for example different polymer phases, at least one of which isbirefringent. In the illustrated exemplary embodiment, the outputcoupling element 528 has scattering elements 528 a, formed of a firstmaterial, embedded within a continuous matrix 528 b formed of a secondmaterial. The refractive indices for the two materials are substantiallymatched for light in one polarization state and remain unmatched forlight in an orthogonal polarization state. Either or both of thescattering elements 528 a and the matrix 528 b may be birefringent.

If, for example, the refractive indices are substantially matched forlight polarized in the x-z plane, and the refractive indices of thefirst and second materials are n₁ and n₂ respectively, the conditionholds that n_(1x)≈n_(1z)≈n_(2x)≈n_(2z), where the subscripts x and zdenote the refractive indices for light polarized parallel to the x andz axes respectively. If n_(1y) # n_(2y), light polarized parallel to they-axis, for example light 530, may be scattered within the outputcoupling element 528 and pass out of the controlled transmission mirror520. The orthogonally polarized light, for example light ray 532,polarized in the x-z plane, remains substantially unscattered on passingwithin the output coupling element 520 because the refractive indicesfor this polarization state are matched. Consequently, if the light 532is incident on the top surface 529 of the output coupling element 528 atan angle equal to, or greater than, the critical angle, θ_(c), of thecontinuous phase 528 b, the light 532 is totally internally reflected atthe surface 529, as illustrated.

To ensure that the light extracted from the output coupling element 528is well polarized, the matched refractive indices are preferably matchedto within at least ±0.05, and more preferably matched to within ±0.01.This reduces the amount of scatter for one polarization. The amount bywhich the light in the y-polarization is scattered is dependent on anumber of factors, including the magnitude of the index mismatch, theratio of one material phase to the other and the domain size of thedisperse phase. Preferred ranges for increasing the amount by which they-polarized light is forward scattered within the output couplingelement 528 include a refractive index difference of at least about0.05, a particle size in the range of about 0.5 μm to about 20 μm and aparticle loading of up to about 10% or more.

Different arrangements of polarization-sensitive output coupling elementare available. For example, in the embodiment of output coupling element548, schematically illustrated in FIG. 5B, the scattering elements 548 aconstitute a disperse phase of polymeric particles within a continuousmatrix 528 b. Note that this figure shows a cross-sectional view of theoutput coupling element 548 in the x-y plane. The birefringent polymermaterial of the scattering elements 548 a and/or the matrix 548 b may beoriented, for example, by stretching in one or more directions. Dispersephase/continuous phase polarizing elements are described in greaterdetail in co-owned U.S. Pat. Nos. 5,825,543 and 6,590,705, both of whichare incorporated by reference.

Another embodiment of polarization-sensitive output coupling element 558is schematically illustrated in cross-section in FIG. 5C. In thisembodiment, the scattering elements 558 a are provided in the form offibers, for example polymer fibers or glass fibers, in a matrix 558 b.The fibers 558 a may be isotropic while the matrix 558 b isbirefringent, or the fibers 558 a may be birefringent while the matrix558 b is isotropic, or the fibers 558 a and the matrix 558 b may both bebirefringent. The scattering of light in the fiber-based, polarizationsensitive output coupling element 558 is dependent, at least in part onthe size and shape of the fibers 558 a, the volume fraction of thefibers 558 a, the thickness of the output coupling element 558, and thedegree of orientation, which affects the amount of birefringence.Different types of fibers may be provided as the scattering elements 558a. One suitable type of fiber 558 a is a simple polymer fiber formed ofone type of polymer material that may be isotropic or birefringent.Examples of this type of fiber 558 a disposed in a matrix 558 b aredescribed in greater detail in co-owned U.S. patent application Ser. No.11/068,159, incorporated herein by reference. Another example of polymerfiber that may be suitable for use in the output coupling element 558 isa composite polymer fiber, in which a number of scattering fibers formedof one polymer material are disposed in a filler of another polymermaterial, forming a so-called “islands-in-the-sea” structure. Either orboth of the scattering fibers and the filler may be birefringent. Thescattering fibers may be formed of a single polymer material or formedwith two or more polymer materials, for example, a disperse phase in acontinuous phase. Composite fibers are described in greater detail inU.S. patent application Ser. Nos. 11/068,157 and 11/068,158, both ofwhich are incorporated by reference.

It will be appreciated that the input coupling element may also bepolarization sensitive. For example, where unpolarized light is incidenton the controlled transmission mirror, a polarization-sensitivescattering input coupling element may be used to scatter light of onepolarization state into the controlled transmission mirror, allowing thelight in the orthogonal polarization to be reflected by the multilayerreflector back to the base reflector. The polarization state of thereflected light may then be mixed before returning to the controlledtransmission mirror. Thus, the input coupling element may permit lightin substantially only one polarization state to enter the controlledtransmission mirror. If the different layers of the controlledtransmission mirror maintain the polarization of the light, thensubstantially only one polarization of light may be extracted from thecontrolled transmission mirror, even if a non-polarization-sensitiveoutput coupling element is used. Both the input and output couplingelements may be polarization sensitive. Any of the polarizationsensitive layers used as an output coupling element may also be used asan input coupling element.

Some different exemplary embodiments of illumination light unit that usea controlled transmission mirror are schematically illustrated in FIGS.6-8. In FIG. 6A, the illumination light unit 600 includes an elongatedreflecting cavity 602 with one or more light sources 606 at one end.FIG. 6B shows a cross section through the light unit 600. There may beone or more light sources positioned at the other end (not shown). Thereflecting cavity 602 is provided with one or more reflective walls 612,so that light from the light source is reflected within the reflectingcavity 602 and is extracted from the cavity 602 via a controlledtransmission mirror 604. In some exemplary embodiments, the interior ofthe reflecting cavity 602 may be empty, in which case the reflectivewalls 612 are formed on the inner surfaces of exterior walls. In otherexemplary embodiments, the reflecting cavity 602 may be formed byproviding reflectors on the outside surface of a solid, transparentbody.

In some embodiments, the reflective walls 612 may be diffuselyreflective and in other embodiments the reflective walls 612 may bespecularly reflective. The reflective walls 612 may be, for example,multilayer dielectric coatings, multilayer polymer optical films, ormetallic coatings. In some embodiments, the reflective walls need notlie parallel to the controlled transmission mirror 604. The reflectivewalls 612 need not completely surround the reflecting cavity 602, butare arranged instead so that at least some of the light is reflectedbetween the reflective walls and the controlled transmission mirror 604.

In some exemplary embodiment the ends 608 may be reflective. In theillustrated embodiment, the reflecting cavity 602 has a circularcross-section, although it will be appreciated that other shapes ofcross-sections may also be used, for example the cross-section may beelliptical, triangular, square, rectangular, or some other shape. Thedimensions of the controlled transmission mirror 604 may be set to matchthe dimensions of the edge of the light guide that is being illuminatedby the illumination light unit 600.

The path of an exemplary light beam 614 is shown in FIG. 6B. The lightbeam 614 is reflected by the reflective walls 612 within the reflectingcavity 602 and is transmitted through the controlled transmission mirror604.

The light source(s) 606 may comprise one or more LEDs. The LEDs may allproduce light of the same color. In another embodiment, the light source606 may include a color converter, such as a phosphor, for generatinglight of a different color from that generated within the LED. Forexample, a phosphor may be used to generate white light using a blue orUV LED. The light source 606 may be located within the reflecting cavity602, for example, if the light source 606 is an LED, then the end 608may be provided with a hole through which the LED is passed from theoutside of the cavity 602 to the inside. In another embodiment, thelight source 606 may be located outside the reflecting cavity 602 andthe light from the light source 606 may pass through an aperture intothe reflecting cavity 602.

The extraction of light through the controlled transmission mirror 604may be graded along its length so that less light is extracted from thereflecting cavity 602 closer to the light source 606, with increasingextraction further away from the light source 606, so that thebrightness of the light extracted from the illumination light unit 600is relatively uniform along its length.

Another embodiment of an illumination light unit 700 is shown in FIG.7A, in which a number of light sources 706 are located at the end 710 ofa reflecting cavity 702. In this exemplary embodiment, there is morethan one light source 706 and the cross-sectional shape of thereflecting cavity 702 is rectangular. The light sources 706 may eachgenerate light of the same color or of a different color. In the casewhere different light sources 706 generate light of different colors,the light from each light source 706 is mixed in the reflecting cavity702 with the light from the other light sources 706 so that the lightemerging from the controlled transmission mirror 704 may be a mixedcolor. For example, if there are three light sources 706 producing red,green and blue light respectively, the light emerging from thecontrolled emission mirror 704 may be a white color. The shade of themixed color output light depends, inter alia, on the relative outputpowers of the different light sources and on the spectral properties ofthe controlled transmission mirror 704.

The extraction of light through the controlled transmission mirror 704may be graded along the length of the controlled transmission mirror 704so that the brightness of the light extracted from the illuminationlight unit 700 is relatively uniform along its length.

An exemplary embodiment of a backlight 720 that uses the illuminationunit 700 is schematically illustrated in FIG. 7B. The illumination unit700 is at least partially surrounded by a reflector 722 and ispositioned so that the light 724 emitted from the controlledtransmission mirror 704 is directed towards a lightguide 726. Anoptional brightness enhancing layer 728, for example a prismaticbrightness enhancing layer, may be positioned between the illuminationunit 700 and the lightguide 726. The brightness enhancing layer 728reduces the angular spread of the light entering the lightguide 726 andmay promote lateral spreading in the lightguide 726. Some of the light,for example ray 730, may be reflected by the brightness enhancing layer728. The reflected light 730 may be redirected back towards thelightguide 726 by the controlled transmission mirror 704 or some otherreflector in the illumination unit 700, or by the reflector 722 thatsurrounds the illumination light unit 700. It will be appreciated thatother embodiments of illumination light unit, for example illuminationunit 600, may also be used in such a configuration for illuminating alight guide.

Another embodiment of an illumination light unit 800 is shown in FIG.8A, in light sources 806 are located on a face 808 of a reflectingcavity 802 opposing a controlled transmission mirror 804. In thisexemplary embodiment, there is more than one light source 806 and thecross-sectional shape of the reflecting cavity 802 is rectangular. Thelight sources 806 may each generate light of the same color or ofdifferent colors. The reflective cavity 802 may be used to mix the lightfrom the different light sources 806 so that the intensity profile ofthe light output from the controlled transmission mirror 804 isrelatively uniform. Furthermore, in the case where the light sources 806produce light of different colors, the different colored light is mixedso that the light emerging from the controlled transmission mirror 804is a mixed color. For example, if there are three light sources 806producing red, green and blue light respectively, the light emergingfrom the controlled emission mirror 804 may be a white color. The lightfrom the light sources 806 may be mixed within the reflecting cavity 802so that the brightness of the light extracted from the illuminationlight unit may be relatively uniform.

Another embodiment of an illumination unit 820 is schematicallyillustrated in FIG. 8B, in which the controlled transmission mirror 854is positioned on the top of the reflecting cavity 802. Additional lightsource 806 may be placed around the edge of the reflecting cavity 802.

Another embodiment of an illumination light unit 900 is schematicallyillustrated in FIGS. 9A and 9B. The unit 900 has a reflecting cavity 902that includes a reflector 908 and a controlled transmission mirror 904.One or more light sources 906 are provided on a base 907. The base 907may be reflective. The base 907 may also provide electrical connectionsfor driving the light source 906 and provide a heatsink for removingheat from the light source 906.

Light 920 from the light sources 906 is reflected by the reflector 908towards the controlled transmission mirror 904. The reflector 908 mayhave any suitable shape and may be curved (as illustrated) or flat. Ifthe reflector 908 is curved, the curve may be any suitable type ofcurve, for example elliptical or parabolic. In the illustratedembodiment, the reflector 908 is curved in one dimension. The reflector908 may be any suitable type of reflector, for example a metalizedreflector, a multilayer dielectric reflector or a multiple layer polymerfilm (MOF) reflector. Light that is transmitted through the controlledtransmission mirror 904 may be coupled into a light guide 912 forback-illuminating a display device. The space 914 within the reflectingcavity 902 may be filled, or may be empty. In embodiments where thespace 914 is filled, for example with a transparent optical body, thenthe reflector 908 may be a reflective coating on the body. Inembodiments where the space is empty, then the reflector 908 may be afront surface reflector. Different configurations of reflective cavityare described further in U.S. patent application Ser. Nos. 10/701,201and 10/949,892, incorporated herein by reference.

The light sources 906, for example LEDs, may all produce light of thesame color, or different LEDs may produce light of different colors, forexample red, green and blue. In some exemplary embodiments, an optionalwavelength converter 922 may be used to change the color of at leastsome of the light 920. For example, where the light 920 is blue orultraviolet, the wavelength converter 922 may be used to convert some ofthe light to green and/or red light 924 (dashed lines). A low-passreflector 926 may be positioned between the controlled transmissionmirror 904 and the wavelength converter 922. The low-pass reflector 924transmits the relatively short wavelength light 920 from the lightsources 906 and reflects light 924 a from the wavelength converter 922towards the light guide 912.

In another embodiment, the controlled transmission mirror 904 may use asan output coupling element a diffuser having a matrix loaded withphosphor particles. In such a configuration, some of the lighttransmitted through the multilayer reflector is converted by thephosphor to light of a different wavelength. Light that is not diffusedor converted by the particles may be totally internally reflected by thematrix layer so as to pass back through the multilayer reflector.

Another embodiment of an illumination light unit 1000 is schematicallyillustrated in FIG. 10. The unit 1000 includes a number of reflectingcavities 1002 formed between curved reflectors 1008 and a controlledtransmission mirror 1004. The controlled transmission mirror 1004 may beprovided as a single sheet common to each of the reflecting cavities1002, or may be provided as a segmented mirror, each segment beingassociated with a respective cavity 1002. One or more light sources 1006are provided in each reflecting cavity. The light sources 1006 may bemounted on a base 1007 that may be reflective. The base 1007 may alsoprovide electrical connections for driving the light sources 1006 andprovide a heatsink for removing heat from the light sources 1006.

Light 1020 from the light sources 1006 may be reflected by thereflectors 1008 towards the controlled transmission mirror 1004. Thereflectors 1008 may have any suitable shape and may be curved (asillustrated). Any suitable type of curve shape may be used for thecurved the reflector 1008, for example ellipsoidal or paraboloidal. Inthe illustrated embodiment, the reflectors 1008 are curved in twodimensions. The reflectors 1008 may be any suitable type of reflector,for example a metalized reflector, a multilayer dielectric reflector oran MOF reflector. Light that is transmitted through the controlledtransmission mirror 1004 may be coupled into a light guide 1012 forback-illuminating a display device.

The light sources 1006, for example LEDs, may all produce light of thesame color, or different LEDs may produce light of different colors, forexample red, green and blue. In some exemplary embodiments, an optionalwavelength converter 1022 may be used to change the color of at leastsome of the light 1020 that passes out of the controlled transmissionmirror 1004. A low-pass reflector 1026 may be positioned between thecontrolled transmission mirror 1004 and the wavelength converter 1022.

Another embodiment of an illumination unit 1100 that may be used as abacklight for a display device is schematically illustrated in FIG. 11A.In this embodiment, one or more light sources 1106 are disposed betweenfirst and second reflectors 1102, 1104. In some embodiments, the lightsources 1106, which may be LEDs, may emit light substantially away fromthe second reflector 1102, in which case an optional curved reflector1108 may be provided to direct the light 1110 along the space betweenthe first and second reflectors 1102, 1104. In other embodiments, notillustrated, the light sources 1106 may substantially emit lightsideways in a direction along the space between the first and secondreflectors 1102, 1104.

The first and second reflectors 1102, 1104 may be specular reflectors,for example ESR film available from 3M Company, St. Paul, Minn. Afolding reflector 1112 is positioned at each end to fold the light 1110into a reflecting cavity formed between the second reflector 1104 and acontrolled transmission mirror 1114. The light 1110 is eventuallydirected out of the unit 1100 through the controlled transmission mirror1114. The first reflector 1102 may be mounted on a base 1116 thatprovides electrical power to the light sources 1106 and may also operateas a thermal sink to remove heat from the light sources 1106.

The light sources 1106 may be arranged in different patterns on thefirst reflector 1102. In the arrangement illustrated in FIG. 11B, whichshows a slice through the unit 1100 between the first and secondreflectors 1102, 1104, the light sources 1106 are arranged in a linearpattern, with the light being directed towards the edges 1120 a, 1120 b.In the arrangement schematically illustrated in FIG. 11C, the lightsources 1106 and reflector 1108 are arranged in a radial pattern, sothat the light is directed radially outwards to the folding reflector1112 situated around the periphery of the first reflector 1102.

An illumination light unit as described herein is not restricted to usefor illuminating a liquid crystal display panel. The illumination lightunit may also be used wherever discrete light sources are used togenerate light and it is desirable to have uniform illumination out of apanel that includes one of more of the discrete light sources. Thus, thecontrolled transmission mirror may find use in solid state spacelighting applications and in signs, illuminated panels and the like.

The present invention should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the present specification. Theclaims are intended to cover such modifications and devices.

1. An illumination light unit, comprising: at least a first light sourcecapable of generating illumination light; and a reflecting cavity havingone or more reflectors and a controlled transmission mirror disposed atan output of the reflecting cavity, the controlled transmission mirrorcomprising an input coupling element, an output coupling element, atransparent layer disposed between the input and output couplingelements, and a first multilayer reflector disposed between the inputand output coupling elements, at least some of the illumination lightfrom the at least a first light source being reflected within thereflecting cavity by the one or more reflectors and being transmittedout of the reflecting cavity through the controlled transmission mirror.2. A unit as recited in claim 1, wherein the at least a first lightsource comprises a light emitting diode (LED).
 3. A unit as recited inclaim 1, wherein the at least a first light source comprises at least afirst light source and a second light source, the first light sourcegenerating light at a first wavelength and the second light sourcegenerating light at a second wavelength different from the firstwavelength.
 4. A unit as recited in claim 1, further comprising a lightwavelength converter disposed to convert the wavelength of theillumination light output through the controlled transmission mirror. 5.A unit as recited in claim 1, wherein the reflecting cavity is elongatedalong a longitudinal axis and has a first end, and the controlledtransmission mirror is on a first side of the reflecting cavity,substantially parallel to the longitudinal axis.
 6. A unit as recited inclaim 5, wherein the at least a first light source is disposed at thefirst end of the reflecting cavity.
 7. A unit as recited in claim 5,wherein the at least a first light source is disposed on a second sideof the reflecting cavity.
 8. A unit as recited in claim 1, wherein thereflecting cavity comprises at least one curved reflector on an opticalpath between the one or more light sources and the controlledtransmission mirror.
 9. A unit as recited in claim 8, wherein the atleast one curved reflector is curved in one dimension only.
 10. A unitas recited in claim 8, wherein the at least one curved reflectorcomprises at least two curved reflectors, each of the curved reflectorscurved in two dimensions.
 11. A unit as recited in claim 1, wherein thetransparent layer is disposed between the first multilayer reflector andthe output coupling element.